Both Raman and Brillouin scattering phenomena have been used for distributed temperature monitoring for many years. Raman was first proposed for sensing applications in the 80's, whereas Brillouin was introduced later as a way to enhance the range of Optical Time Domain Reflectometry (OTDR) and then for strain and/or temperature monitoring applications.
Optical fiber sensors based on Brillouin scattering have been used extensively in the measurement of distributed temperature and/or strain. Both the frequency shift and the power of the Brillouin backscatter signal are dependent on temperature and strain. Brillouin scattering can be used in both a stimulated and spontaneous mode for distributed sensors.
Spontaneous scattering uses one laser light at stable wavelength (optical frequency) and measures spectrum of the backscattered light. It has an advantage that there is no need for modulation to sweep in optical frequency, and that the fiber is single ended. The resulting simplicity is a great benefit.
However, it comes with the disadvantage of a low dynamic range. In order to perform spectrum analysis, the detection scheme can become complicated. Further, because the backscattered signal is very weak, the signal-to-noise ratio will be low and it will require long integration time, high number of measurements for averaging, or both.
Stimulated systems are either double ended or make use of a reflective mirror at the end of the fiber coupled with a counter propagating arrangement. Stimulated scattering requires two input lights (probe and pulse), and at least one of them needs to be modulated and swept across optical frequency bandwidth (10-14 GHz). Further, the two input lights need to be counter-propagating in order to produce stimulated scattering, so most of the work in this area have been based on a dual-ended scheme. This gives a much larger dynamic range. A major disadvantage with such a stimulated system is that if a fiber break occurs, the system is lost.
A growing field is the use of Fiber Bragg gratings (FBG's). The physical principle behind the FBG sensor is that a change in strain, stress, or temperature will alter the center of the wavelength of the light reflected from an FBG. A fiber's index of refraction depends on the density of the dopants it contains. FBGs are made by redistributing dopants to create areas that contain greater or lesser amounts, using a technique called laser writing. The FBG wavelength filter consists of a series of perturbations in the index of refraction along the length of the doped optical fiber. This index grating reflects a narrow spectrum that is directly proportional to the period of the index modulation (L) and the effective index of refraction (n).
Because the temperature and strain states of FBGs directly affect their reflectivity spectrum, they can also be used for a variety of sensing applications. As the fiber-optic analogue to conventional electronic sensors, FBGs can serve as strain-gauge sensors to provide structural engineers with measurements not previously possible. Emerging applications include detecting changes in stress in buildings, bridges, and airplane bodies; depth measurements in streams, rivers, and reservoirs for flood control; and temperature and pressure measurements in deep oil wells. The advantages of FBG sensors include: improved accuracy, sensitivity, and immunity to electromagnetic interference, radio-frequency interference, and radiation; the ability to be made into a compact, lightweight, rugged device small enough to be embedded or laminated into structures or substances to create smart materials that can operate in harsh environments—such as underwater—where conventional sensors cannot work; the ability to be multiplexed; ease of installation and use; and potential low cost as a result of high-volume telecommunications manufacturing
These features enable using many sensors on a single optical fiber at arbitrary spacing. Using tunable lasers, one can interrogate each sensor independently and obtain a distributed measurement over large structures. Because the gratings are multiplexed on a single fiber, many sensors can be accessed with a single connection to the optical source and detector. Conventional electronic strain gauge sensors require each sensor to have its lead wires attached and routed to the sensor readout. In the application to be discussed the use of spaced FBG's is used in a novel way to achieve a substantial improvement in system reliability in a Brillouin system.
There is a need for a system with the benefits of both a single ended spontaneous system as well as the improved dynamic range of a stimulated Brillouin system.